A typical hot melt dispenser provides for fluid flow through a body cavity and then out a nozzle orifice toward a substrate or work-piece. A needle valve located inside the dispenser body seats within a valve seat formed by the nozzle orifice, thereby preventing flow of the dispensing fluid out of the nozzle orifice. Movement of the needle valve away from the valve seat permits fluid flow out of the nozzle orifice at a rate commensurate with the dimensions of the gap between the needle valve and the valve seat.
It is desirable to accurately and adjustably control the flow of dispensing fluid through the nozzle orifice. For a given gap size between the needle valve and the valve seat, the dispensing rate will vary with the viscosity of the material being dispensed. To accurately control dispensing rate for materials having a wide range of viscosities without requiring time consuming nozzle changes, it is desirable for a fluid dispenser to provide reliable and repeatable control and adjustment of the gap between the needle valve and the valve seat.
Typically, the needle valve for dispensers of this type carries a piston at an end opposite the valve seat. The piston is centered within the body, and the application of pressurized air on one face of the piston causes the needle valve to move away from the valve seat.
To set a maximum gap between the needle valve and the valve seat, i.e., a maximum stroke distance, a stop member is housed within the body, in alignment with the piston. The stop member limits the travel distance of the piston, and hence the gap between the needle valve and the valve seat during dispensing. By providing for adjustment of the position of the stop member with respect to the piston, the maximum gap between the needle valve and the valve seat may be varied. Thus, adjustment of the position of the stop member enables a single dispenser to be used with materials which vary in viscosity.
To prevent undesired dispensing of fluid and to maintain maximum control of fluid dispensing, the needle valve is biased to a closed position. This biasing force is usually supplied by a compression spring which acts upon a face of the piston opposite the face acted upon by the pressurized air. The biasing force applied to the piston determines the closing force of the needle valve when dispensing ceases. It is necessary for the spring to maintain a sufficiently high closing force on the piston to effectively terminate dispensing with little or no dripping. Because the closing force will vary with the viscosity of the material being dispensed, it is also desirable to provide for adjustment of this applied biasing force, i.e. the spring load.
Thus, it is desirable to provide adjustability for the stop member of a fluid dispenser to control the stroke distance of the needle valve, thereby to achieve a desired flow rate for materials of varying viscosity. At the same time, it is also desirable to provide adjustability for the spring load used to bias the needle valve to a closed position.
Applicant's U.S. Pat. No. 4,801,051, entitled "Flow Control Devise for Fluid Dispensing Apparatus", and which is expressly incorporated by reference herein in its entirety, discloses a device which provides for microadjustment of the stroke length of a hot melt dispensing module. This device provides for adjustment of both stroke length an closing force to enable a single dispensing gun to be used with materials of different viscosities.
However, because of the structural arrangement of the stop member and the compression spring of this dispenser, adjustment of the position of the stop member also affects the spring load, while adjustment of the spring load affects the stroke length. As a result, it is difficult to simultaneously quantify both of these parameters, i.e., closing force and gap size, to achieve predetermined and predictable results in dispensing fluids of varying viscosity from a single dispensing gun. Typically, after changing over to a different dispensing fluid, an operator must run several tests to verify that the adjustable settings of stroke length and spring load produce the desired result.
It is an object of this invention to eliminate guesswork commonly associated with setting and controlling the gap size and the closing force of a needle valve of a fluid dispenser.
It is another object of this invention to enhance accuracy and predictability in dispensing fluids of varying viscosity from a single liquid dispenser.
This invention meets the above-stated objectives by structurally isolating compression spring adjustment components from stop member adjustment components within the body of a fluid dispenser so that the closing force and the gap size of the needle valve can be independently controlled.
Because this invention provides for independent control of the closing force and the gap size of a fluid dispenser, these two parameters may be accurately quantified and repeated so that optimum dispensing is achieved with a single dispenser, regardless of variation in the viscosities of the materials dispensed.
This invention eliminates the guesswork formerly associated with simultaneous control of both closing force and gap size. The fluid dispenser of this invention enables an operator to select, for a material of known viscosity, both a desired gap size, i.e. determined by stroke length of the needle valve, and a desired closing force, i.e., determined by the biasing force applied to the needle valve by the compression spring.
To achieve independent control of both closing force and gap size, this invention contemplates an adjustable stop member aligned axially with the needle valve. Adjustment of the axial position of the stop member with respect to the radial center of a piston carried by the needle valve adjusts the maximum stroke distance of the needle valve, thereby setting the gap between the nozzle orifice and the needle valve.
A compression spring and the components which control the compression applied by the spring are located radially outside of the stop member, so that the spring acts upon the piston radially outside of, and independently of the stop member. The stop member extends axially through the compression spring and a cylindrical retainer which holds the compression spring in place.
According to a preferred embodiment of the invention, a fluid dispenser includes a body with an axial bore therethrough which terminates in a nozzle orifice. A needle valve located within the dispenser is aligned along the axis of the bore, and the needle valve carries a piston which is acted upon by pressurized air to move the needle valve away from the nozzle orifice to dispense fluid therethrough. A stop member is axially aligned with the center of the piston to limit travel of the needle valve away from the nozzle orifice. An upper end of the stop member threadably connects to an upper section of the body, which is bolted to a lower section of the body. Threadable adjustment of the stop member with respect to the piston varies the maximum stroke length of the needle valve.
A compression spring has one end which bears directly against the piston, outside of the stop member, and a cylindrical retainer located within the bore has a lip which bears against the opposite end of the spring. The retainer lip defines a circular passage through which the stop member extends. An adjustment screw spaced away from and parallel with the bore threadably connects to the body. The adjustment screw carries a washer which contacts the retainer to hold it in place. The axial position of the washer determines the axial position of the retainer with respect to the body, and the distance between the retainer lip and the piston determines the axial compression of the spring, and hence the compressive force applied to the piston by the spring. Adjustment of this screw adjusts the spring bias on the piston, thereby setting the closing force for the needle valve.
Because the stop member extends through the compression spring and the cylindrical retainer, adjustment of the axial position of the stop member, and thus the stroke length, does not affect the spring load applied to the piston. Moreover, because the spring and the retainer circumscribe the stop member, adjustment of the axial position of the retainer varies the spring load but does not affect the stroke distance of the needle valve.
Thus, this invention provides independent adjustment of the applied spring load and the stroke length of a needle valve of a liquid dispenser, thereby to improve accuracy in setting a desired closing force and a desired gap size for the needle valve during fluid dispensing. As a result of the independent adjustment capability for both stroke length and spring bias, a single dispenser may be used to accurately and repeatably dispense materials of varying viscosity.
These and other features of the invention will be more readily understood in view of the following detailed description and the drawings.